Confidential manuscript submitted to Geophysical Research Letters 1 On the ubiquity of magnetic reconnection inside flux transfer event-like structures at 2 the Earth’s magnetopause 3 N. Fargette1, B. Lavraud1, M. Øieroset2, T. D. Phan2, S. Toledo-Redondo3,1, R. Kieokaew1, C. 4 Jacquey1, S. A. Fuselier4,5, K. J. Trattner6, S. Petrinec7, H. Hasegawa8, P. Garnier1, V. Génot1, Q. 5 Lenouvel1, S. Fadanelli1, E. Penou1, J.-A. Sauvaud1, D. L. A. Avanov9, J. Burch4, M. O. 6 Chandler10, V. N. Coffey10, J. Dorelli9, J. P. Eastwood11, C. J. Farrugia12, D. J. Gershman9, B. L. 7 Giles9, E. Grigorenko13, T. E. Moore9, W. R. Paterson9, C. Pollock14, Y. Saito8, C. Schiff9, S. E. 8 Smith15 9 10 1 Institut de Recherche en Astrophysique et Planétologie, CNRS, UPS, CNES, Université de 11 Toulouse, Toulouse, France 12 2 Space Sciences Laboratory, University of California, Berkeley, CA, USA 13 3 Department of Electromagnetism and Electronics, University of Murcia, Murcia, Spain 14 4 Southwest Research Institute, San Antonio, TX, USA 15 5 Department of Physics, University of Texas at San Antonio, San Antonio, TX, USA 16 6 Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, 17 USA 18 7 Lockheed Martin Advanced Technology Center, Palo Alto, CA, USA 19 8 Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan 20 9 NASA Goddard Space Flight Center, Greenbelt, MD, USA 21 10 NASA Marshall Space Flight Center, Huntsville, AL, USA 22 11 The Blackett Laboratory, Department of physics, Imperial College London, London, UK 23 12 University of New Hampshire, Durham, NH, USA 24 13 Space Research Institute of the Russian Academy of Sciences, Moscow, Russia 25 14 Denali Scientific, Fairbanks, AK, USA 26 15 Catholic University of America, Washington, DC, USA 27 Corresponding author: Naïs Fargette ([email protected]) 28 Key Points: 29 x 19% of FTE-type structures observed by MMS ͳ ͳ present 30 signatures of magnetic reconnection in their core. 31 x They seem to be formed by two magnetically disconnected interlaced flux tubes and are 32 typically observed for large IMF ܤ௬. 33 x Several formation models are discussed, including a bifurcated X-line scenario that 34 results from the maximum shear angle model. 1 Confidential manuscript submitted to Geophysical Research Letters 35 Abstract 36 Flux Transfer Events (FTEs) are transient phenomena frequently observed at the Earth’s 37 magnetopause. Their usual interpretation is a flux rope moving away from the reconnection 38 region. However, the Magnetospheric Multiscale Mission revealed that magnetic reconnection 39 sometimes occurs inside these structures, questioning their flux rope configuration. Here we 40 investigate 229 FTE-type structures and find reconnection signatures inside 19% of them. We 41 analyze their large-scale magnetic topology using electron heat flux, and find it is significantly 42 different across the FTE reconnecting current sheets, demonstrating they are constituted of two 43 magnetically disconnected structures. We also find that the Interplanetary Magnetic Field (IMF) 44 associated with reconnecting FTEs presents a strong ܤ௬ component. We discuss several 45 formation mechanisms to explain these observations. In particular, the maximum magnetic shear 46 model predicts that for large IMF ܤ௬, two spatially distinct X-lines coexist at the magnetopause. 47 They can generate separate magnetic flux tubes that may become interlaced. 48 Plain language summary 49 The solar wind and the Earth’s magnetosphere are two gigantic magnetic structures that collide 50 constantly over our heads, in the near space environment. At the boundary of their interaction 51 (the magnetopause), the fundamental process of magnetic reconnection can occur. It is there that 52 dynamic magnetic structures called ‘Flux Transfer Events’ are formed. They travel fast along the 53 magnetopause and transport a lot of energy, from the solar wind into the magnetosphere. These 54 structures are yet not well understood, as underlined by the recent observations made by the 55 Magnetospheric Multiscale Mission (MMS), launched in 2015 by NASA. The four-spacecraft 56 mission, specifically designed to study the physics happening at the magnetopause, is capable of 57 measuring right into these magnetic structures, collecting data on their particles and magnetic 58 field properties. When analyzing MMS data, we found that 19% of the ‘Flux Transfer Event’ 59 were not constituted of one, but two structures with very different properties. These dual 60 magnetic structures tend to appear when the solar wind’s magnetic field is oriented mainly 61 toward the east or the west. From these observations and based on existing models of the 62 magnetopause, we propose a scenario that allows such dual structures to form as interlaced 63 magnetic tubes. 64 1 Introduction 65 Flux Transfer Events (FTEs) are transient phenomena that frequently occur at the Earth’s 66 dayside magnetopause, resulting from the dynamic interaction of the solar wind with the 67 magnetosphere. In the early model of Russell and Elphic (1979), they result from bursty and 68 patchy magnetic reconnection, and consist in elbow flux tubes moving away from the subsolar 69 region. Two main models were later proposed. The first one (Southwood et al. (1988); Scholer 70 (1988)) is based on a single spatially stable X-line at the subsolar magnetopause, but whose 71 reconnection rate varies over time. This time variation leads to the formation of magnetic field 72 bulges that are identified as FTEs. The other main model is the multiple X-line scenario (Lee and 73 Fu (1985)), relying on two X-lines appearing sequentially on the magnetopause. As the first X- 74 line forms and then drifts towards the poles, a second X-line reforms near the equator (and 75 remains connected to the first one). The FTE is then the structure trapped in between these two 76 reconnection lines. Over the years, many studies have been conducted to discriminate between 77 these formation models through simulations (e.g. Fedder et al. (2002); Raeder (2006)) and multi- 2 Confidential manuscript submitted to Geophysical Research Letters 78 spacecraft observations (e. g. Hasegawa et al. (2006; 2010); Farrugia et al. (2016)). The literature 79 to date suggests a paradigm such that the multiple X-line model is the predominant FTE 80 formation mechanism. In all of these views, FTEs resemble flux ropes, as they are thought of as 81 three-dimensional helical structures. Their expected in situ signatures are primarily an 82 enhancement in their core magnetic field strength and a bipolar signature in the magnetic field 83 component normal to the magnetopause surface. 84 The Magnetospheric Multiscale Mission (MMS, Burch et al. (2016)) with its high- 85 resolution instrumentation has unveiled new features of FTEs. In particular, structures that look 86 like classical FTEs have been reported to display reconnection signatures in their center, with 87 clear ion jets correlated with a thin current sheet. While thin current sheets inside FTEs had 88 previously been observed with the THEMIS missions (e.g. Hasegawa et al. (2010); Øieroset et 89 al., (2011)), only the recent MMS measurements enabled us to confirm that magnetic 90 reconnection was occurring (Øieroset et al. (2016; 2019); Kacem et al. (2018)). Detailed studies 91 of such events led to the conclusion that these structures did not match the regular magnetic flux 92 rope configuration, but rather consisted of interlaced flux tubes such that the reconnecting 93 current sheet separates two magnetically disconnected regions (Kacem et al. (2018), Øieroset et 94 al. (2019)). The interpretation of some FTE-type phenomena as complex 3D structures with 95 interlaced flux tubes was first proposed by Nishida (1989) and Hesse et al. (1990). It was studied 96 through simulations (Lee et al. (1993), Otto (1995), Cardoso et al. (2013), Farinas Perez et al. 97 (2018)) and observed in Cluster data (Louarn et al. (2004)) prior to MMS. 98 In this paper we study statistically the FTEs observed by MMS, investigating in more 99 depth the occurrence and implications of the reconnection signatures found inside FTEs. We find 100 that 19% of FTEs present these signatures in their core, and are consistent with the magnetically 101 disconnected flux tube structure similar to Kacem et al. (2018). We also find that the 102 Interplanetary Magnetic Field (IMF)’s orientation plays a significant role in the formation of 103 such structures. 104 2 Data 105 We use data measured by the four-spacecraft MMS mission throughout phase 1, from 106 2015 to 2017. The magnetic field data is acquired by the Fluxgate Magnetometer (Russell et al. 107 (2016)), with a 128Hz time resolution and a 0.1nT precision. The ion and electron distribution 108 functions and associated moments are acquired by the Fast Plasma Investigation instruments 109 (Pollock et al. (2016)). Only burst mode data is used, giving a time resolution of 30ms for 110 electron measurement and 150ms for ions. Data is presented in the Geocentric Solar Ecliptic 111 (GSE) coordinate system and taken from the MMS1 spacecraft. 112 We also obtain solar wind conditions from the OMNI database (King and Papitashvili 113 (2005)). 114 3 Selection process 115 Although our selection process tries to be as objective as possible, part of it relies on data 116 visual inspection and thus is susceptible to subjectivity. For reproducibility purposes, the 117 auxiliary material contains timetables of all selected events. 3 Confidential manuscript submitted to Geophysical Research Letters 118 3.1 FTE Selection 119 To build the FTE database, we examined all the events listed as potential FTEs and flux 120 ropes by the Scientists In The Loop (SITL) of the MMS mission for phase 1A and 1B. We 121 discarded events that were eventually not FTEs (e.g., magnetopause crossings, bow shock 122 crossings or other associated features).